The invention relates to the field of voltage source converters and especially multilevel converters. Voltage source converters (VSC) have changed power transmission and distribution and using power electronics including semiconductor switching elements that can be turned off, such as IGBTs (Insulated Gate Bipolar Transistors) have found great use for DC transmission, reactive power compensation, control of active as well as reactive power, being able to create AC voltage out of DC voltage by means of switching control, and for converting AC to DC etcetera.
The multilevel converter technique, employing many voltage levels, wherein each voltage level being individually switched, can be used to create AC voltage from DC in small voltage steps providing a stepped voltage curve much closer to a sinus curve than the previous use of two level and three level converters. Often, the energy storage means used consists of capacitors but may also be batteries.
A problem that may arise is that voltages over individual energy storage means become too large or too low.
U.S. Pat. No. 5,532,575 (D1) describes a multilevel converter with means for balancing voltages of capacitors of the converter. D1 describes a multilevel converter primarily intended for use as a static VAr compensator (column 1, line 5-8). The multilevel converter includes three legs, one for each phase, of switching elements (GTO's 30, see FIG. 1), which switching elements (GTO's) are connected to tapping points of capacitors 20 (column 1, line 28-34). The multilevel converter also includes a control system 60 (column 7, line 48-65) that controls the switching of the GTO's. The control system monitors the voltages of the capacitors and (see column 8 line 32-64) if a voltage level of a capacitor is too high or too low, the control system (see abstract) adjusts the timing of the switching of those capacitors that have too low or too high voltage level, but do not change the switching timing of those capacitors that do not deviate. In this way the voltages of those capacitors that do not deviate is not affected (column 8, line 39), whereas the voltages of the deviating capacitors are balanced.
A document that describes a similar topology and switching control in a multilevel converter for a different purpose is U.S. Pat. No. 6,088,245 (D2). D2 describes a switching control arrangement for multilevel converters that counteract the harmonic content of the converter voltage or current by controlling the switching pattern of the switching devices, e.g. GTO's (see abstract). Especially, the switching pattern is changed by modifying the timing of the switching of the switching devices.
Thus, documents D1 and D2 describe two different goals achieved by adjusting the timing of the switching of the switching devices of a multilevel converter, i.e. balancing capacitor voltages and reducing harmonics, respectively. In the multilevel converters described in D1 and D2, the three phases have common energy storage elements, i.e. the three phase share capacitors.
Another known type of multilevel converters, are converters having a semiconductor switching element in a switching cell circuit having a half bridge or full bridge configuration. For example, two IGBTs are used in each switching cell in a half bridge configuration with a DC capacitor as energy storage element, and each IGBT is arranged in anti-parallel with its own diode.
In such multilevel converters that have separate energy storage elements for each phase, e.g. capacitors that belong to one phase, sharing of energy between the capacitors within a phase leg, or between capacitors of different phase legs, is difficult to achieve without affecting the power that is transferred to the power network.
Document WO2010/145706 (D3) provides a solution for balancing voltages of the energy storage elements of a delta connected multilevel converter, having serially connected switching cells with a corresponding energy storage element, arranged in three phase legs. In more detail, D3 describes a multilevel converter having delta connected phase legs and wherein the DC voltages of the switching cells of each of the phase legs are balanced by means of a balancing current circulating between the phase legs, and distributing energy between the energy storage elements of the phase legs. D3 describes an arrangement for exchanging power in a shunt connection with a three phase power network, which arrangement comprises a voltage source converter having three phase legs in a delta connection, wherein each leg comprises a series of switching cells (see abstract of D3). The electrical conditions of the three phases of the power network and the converter are measured and a control unit (19) is configured to determine if the phases are unbalanced. The control unit (19) determines a zero sequence current that indicates such an unbalance and uses this determined zero sequence current to control the switching cells to add a circulating current to the currents in the phase legs to counteract such an unbalance (see claim 1 of D3). The circulated current is driven inside the delta of the converter legs and moves energy inside the delta, between the legs without negatively affecting the power network, and avoids creating harmonics in the power network (see D3 page 4, lines 24-29).
In such a delta connected multilevel converter the phase legs handle the phase voltage and comprise a sufficient number of levels to handle the voltage level between the phases. A multilevel converter having phase legs connected in a star- or wye-topology would only need a sufficient number of levels to handle the line voltage between ground and the phase. Thus, a disadvantage with a delta topology compared to a wye-topology is that the number of levels needed is larger for handling the higher voltage differences. On the other hand, a disadvantage of making a wye connected converter legs in a multilevel converter having switching cells and corresponding energy storing elements is that using currents to move energy between the energy storing elements affects the power transmission network, since the three legs do not provide a closed circuit as in a delta connected multilevel converter.